Rockets are complicated and expensive objects, in part because they must be very reliable to ensure safety to passengers (for manned vehicles) and to prevent damage to multi-million-dollar payloads, and in part because they must have relatively high performance (e.g., high specific impulse and/or high thrust-to-weight) to allow the rocket mission to be performed within one to three stages, as additional stages typically reduce reliability. Therefore, even the simplest rocket engine design, a solid propellant engine, may be prohibitively expensive.